The invention pertains to 2-dimensional touch sensing surfaces operable by a human finger, or a stylus. Example devices include touch screens and touch pads, particularly those over LCDs, CRTs and other types of displays, or pen-input tablets, or encoders used in machinery for feedback control purposes. In particular this invention pertains to 2-dimensional capacitive touch sensing surfaces constructed so that the sensing layer is disposed on the rear of a panel or lens surface, particularly for use in smaller touch screens where there is a space constraint along the edges of the screen, for example in portable devices such as mobile phones or handheld media players. In addition the invention addresses the need to reduce the effects of capacitive ‘hand shadow’.
In my earlier U.S. application Ser. No. 10/916,759 (published as US2005/0041018A), there is a pattern of galvanically coupled conductors which have anisotropic galvanic properties within the sensing region, due to the use of conductive stripes which prevent current flows in more than one direction, or possibly through the use of a special unpatterned anisotropic conductive material. At least four connections are made, one at each of the corners of the sensing layer to a capacitive sensing circuit which detects the signals associated with finger touch. A processor mathematically computes the centroid location of touch within the area using ratiometric methods. A simple quadratic equation or other method corrects for pin-cushion distortion that appears on only two sides of the sensing region.
In my U.S. provisional application 60/745,583 and co-pending U.S. patent application Ser. No. 11/734,813 derived therefrom (not yet published), there is described a hybrid pattern of electrodes, galvanically connected along a first axis and galvanically isolated along a second axis, which have a resistively derived field distribution on the one axis and a capacitively derived field distribution on the second axis. There are a plurality of connections with a resistive conductive path between them on each of two sides of the touch area, for a total of at least four sensing circuit connections to the device. An improvement comprises the use of more than two connections along each resistive path in order to reduce the effects of capacitive hand shadow and to improve spatial resolution along the axis of the resistive elements. There is no inherent distortion in the electrical response of the sensing area which might require mathematical correction. A first ratiometric computation is used to compute position on one axis and a second ratiometric computation to derive touch position on the second axis.
The term ‘two-dimensional capacitive transducer’ or ‘2DCT’ will be used throughout to refer to touch screens, touch sensing pads, proximity sensing areas, display overlay touch screens over LCD, plasma, or CRT screens or the like, position sensing for mechanical devices or feedback systems, or other types of control surfaces without limitation, having a surface or volume capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
The term ‘two-dimensional resistive transducer’ or ‘2DRT’ refers to touch screens or pen input devices based on purely galvanic principles, and known in the industry generically as ‘resistive touch screens’. The term ‘2DxT’ refers to elements of either the 2DCT or 2DRT type.
The term ‘touch’ throughout means touch or proximity by a human body part or mechanical component of sufficient capacitive signal strength to generate a desired output. In the sense of ‘proximity’, touch can also mean to ‘point’ at a 2DCT without making physical contact, where the 2DCT responds to the capacitance from the proximity of the object sufficient to react with a desired effect.
Unless otherwise noted, the term ‘electrode’ refers to a capacitive field-emitting sensing structure or element within the active region of the sensing surface. The term ‘connection’ refers to galvanic contact between the sensing electrodes and the sensing circuitry. The terms ‘object’ and ‘finger’ are used synonymously in reference to either an inanimate object such as a wiper or pointer or stylus, or alternatively a human finger or other appendage, any of whose presence adjacent the element will create a localized capacitive coupling from a region of the element back to a circuit reference via any circuitous path, whether galvanically or non-galvanically. ‘Dielectric’ means any substantially non-conducting material such as plastic, glass, mineral, wood or other substances, particularly in reference to a layer interposed between the electrodes and the object such as a cover panel or film or lens. The term ‘touch’ means any capacitive or galvanic coupling between an object and the electrodes and includes either direct physical contact between an object and the sensing electrodes, or physical contact between object and a dielectric existing between object and the sensing electrodes, or, non-contact coupling to the sensing electrodes which may or may not include an intervening layer of dielectric between the object and the electrodes. The mention of specific circuit parameters, or orientation is not to be taken as limiting to the invention, as a wide range of parameters is possible using no or slight changes to the circuitry or algorithms; specific parameters and orientation are mentioned only for explanatory purposes.
Many types of 2DCT are known to suffer from a geometric distortion known as ‘pin-cushion’ whereby the reported coordinate of touch is in error due to electrical effects on the sensing surface. These effects are described in more depth in various other patents for example in Pepper U.S. Pat. No. 4,198,539. An excellent summary of the known causes, solutions, and problems of the solutions to geometric distortion can be found in a reading of Babb et al, in U.S. Pat. Nos. 5,940,065 and 6,506,983. U.S. Pat. No. 5,940,065 describes succinctly the two major classes of correction: 1) electromechanical methods involving design of or modification to the sensing electrode(s) or its connections; 2) the use of mathematical formulae to correct the distortion.
Touch screen designs exist which use structured sensing electrodes to suppress distortion at the expense of complexity of the sensing element and drive electronics. One example is U.S. Pat. No. 5,844,506 (Binstead) which discloses fine wire electrodes in an x-y arrangement. Another is U.S. Pat. No. 5,650,597 (Redmayne) which discloses an array of unidirectional bars with multiplexed connections. U.S. Pat. No. 6,297,811 (Kent et al.) describes the use of triangular outline shapes each connected to an individual sensing channel. U.S. Pat. No. 4,550,221 (Mabusth) is an example of a matrix approach where x and y oriented electrodes must cross each other, forcing the electrode structure to occupy two or more layers which adds considerable expense and in the case of LCD touch screens reduces transparency. Co-pending U.S. Provisional Application 60/697,613 (published as GB2428306) further describes a method for structuring electrodes which avoid crossovers in the sensing region with little or no distortion, but this method still requires a relatively large number of connections. These methods can all provide good resolution but require a large number of connections and are therefore costly to implement. Also, a high connection count limits utility in smaller touch screens were there is little space surrounding the sensing area to permit large numbers of wiring traces to the connections.
2DCT devices which employ matrix or electrode-to-electrode coupling approaches such as U.S. Pat. No. 4,198,539 (Pepper) or U.S. Pat. No. 5,650,597 (Redmayne) also have a limited ability to project fields through thick materials, or to project their fields slightly into free space to create a ‘point screen’. In the case of U.S. Pat. No. 4,198,539 (Pepper) the individual electrodes are very narrow and as a result have limited surface area which is essential to project a field through a thicker dielectric; as a result, such designs are typically limited in application to track pads for notebook computers and the like, with a thin overlay on top of electrodes. In the case of U.S. Pat. No. 5,650,597 (Redmayne) the fields are closed between adjacent electrode stripes, restricting the field lines to a short path with little field remaining to emerge from the dielectric. Such limitations reduce touch signal strength and prevent the use of the electrodes with thick dielectric layers.
2DCT devices which employ electrodes which are reliant on the resistance of the electrodes, such as my co-pending U.S. application Ser. No. 10/916,759 (published as US2005/0041018A) or U.S. Pat. Nos. 4,198,539 and 5,650,597 are subject to nonlinearities or unit-to-unit inconsistencies which require a calibration process to correct. Such a calibration process adds to the cost of the sensing element and can easily fall out of calibration with time and environmental conditions, such as temperature or humidity or exposure to light, which over time can alter the resistance of the electrodes. The need for recalibration of the sensing element is a serious commercial disadvantage.
It should be noted that the electronic sensing circuitry and methods described in my prior patents and patent applications, i.e. U.S. Pat. Nos. 5,730,165, 6,288,707, 6,466,036, 6,535,200, 6,452,514, and co-pending applications U.S. Ser. No. 10/697,133 (published as US 2004/104826A1 and granted as U.S. Pat. No. 7,148,704) and U.S. Ser. No. 10/916,759 (published as US2005/0041018A) can be used in conjunction with the invention described herein, but, these circuits and methods are not to be construed as limiting. A variety of capacitive sensing circuits and interpretive logic can be used with the invention, to drive the electrodes and to generate the required outputs.
In my co-pending U.S. application Ser. No. 10/697,133 (published as US 2004/104826A1 and granted as U.S. Pat. No. 7,148,704), there is described in conjunction with FIG. 12 a method of using individual resistive 1D stripes to create a touch screen. These stripes can be read either in parallel or sequentially, since the connections to these stripes are independent of one another. Furthermore, in connection with FIG. 6 there is described an interpolated coupling between adjacent elements and an object such as a finger. U.S. Ser. No. 10/697,133 (published as US 2004/104826A1 and granted as U.S. Pat. No. 7,148,704) is incorporated herein by reference.
The present invention is similar to my U.S. Pat. No. 6,288,707 FIG. 4. In this configuration there are interleaved triangular electrodes which resolve position on a first axis, and graduated bar electrodes which resolve position on a second axis. In U.S. Pat. No. 6,288,707 the triangles are wired in two opposing sets, and connected to one sensing channel per set. The rectangular bars are also wired in two sets, with the width ratio of the bars varying as a percentage of a fixed total height along a second axis. Four connections are required for this configuration.
The use of triangular shaped electrodes to create field gradients has been known for some time; for example, U.S. Pat. No. 4,087,625 (Dym et al.) which is a pen input device uses repeating sets of triangular electrodes 14a, 14b to generate capacitive fields. Resistor dividers 27, 32 generate a field gradient on one axis while the triangles generate a gradient on a second axis. This disclosure does not teach the use of a capacitive sensing function in the electrode array itself, relying instead on a pen to pick up the fields. The device cannot therefore detect a human touch.
U.S. Pat. No. 4,659,874 (Landmeier) discloses a pen input device having similar triangular pattern sets which change dimension along one axis. The x dimension is resolved by the field gradient produced by the triangular shapes while the y dimension is resolved by changing the base width of the triangles along the y axis. The invention requires the use of an active pen to inject signals into the sensing array and is not responsive to human touch. One disadvantage of the y axis field gradient produced by this design is that the gain along the vertical axis is insufficient to provide full scale readings, thus requiring the output to be resealed accordingly. Also it is quite difficult to prevent the response from being granular as the patterns can be quite large to accomplish the desired gradient effect on the y axis.
U.S. Pat. No. 4,999,462 (Purcell) discloses a method using triangles, again with a pen input device and wiring from a microcomputer to each individual triangle wherein the pen picks up the electric field from the triangular electrodes. This system is also incapable of detecting human touch.
U.S. Pat. No. 4,705,919 (Dhawan) and U.S. Pat. No. 4,952,757 (Purcell) disclose further pen input devices using triangles.